The ability of phagocytic and lymphoid cells, including the macrophage, polymorphonuclear leukocyte (PMN), and B lymphocyte, to recognize and to bind to immune complexes (IC) has been attributed to the interaction between an IC bound form of the third component of complement (C3) and cell surface receptors specific for this protein. We are attempting to measure directly the equilibrium and kinetic parameters of the binding between activation products of human C3 and their receptors on human blood cells. The activation fragments C3b, C3c, and C3d have been generated by digestion with porcine elastase and subsequently purified. Each fragment has been radiolabeled by reductive methylation using formaldehyde and (3H) sodium borohydride. The interaction with erythrocytes and different leukocyte populations will be assessed using a direct binding assay where equilibrium saturation, time course, and competition experiments are expected to characterize the physio-chemical nature of these interactions. In addition, specific antibodies have been raised in sheep and rabbits to each of these fragments and will be used for (a) probes to identify receptor binding sites in C3b and (b) developing specific radioimmunoassays to assess in vivo complement activation.